Although uniformitarian views dominated early thinking of ocean chemical evolution over geologic time, today we recognize that the composition of seawater has varied significantly over Earth's history. Some changes are ingrained in our thinking (for example, that the Archean ocean was anoxic and iron-rich) while others are rarely considered. For example, if sulfate was a trace constituent of the Archean ocean, then the chemistry of hydrothermal fluids would have been significantly different (more reduced, with high hydrogen partial pressures and iron concentrations but low concentrations of hydrogen sulfide); this may be of significance to those considering such environments as the locus for the origin of life and for early ecosystems. Refinement of radiometric ages of banded iron formations suggest that their deposition was episodic, not continuous, and this may require us to rethink the notion of a persistently Fe-replete Archean ocean. The rise of atmospheric oxygen in the earliest Proterozoic ironically created the potential for highly reducing marine conditions with free hydrogen sulfide in the upper water column supporting anoxygenic phototrophs. The persistence of these conditions through the Proterozoic is uncertain, but when they occurred, the "pale blue dot" may have been pink. Strategies for life detection on distant planets is based in part on our interpretation of Earth's oceanic and atmospheric evolution, and we have some way to go before we can confidently describe the evolutionary history and persistence of particular conditions on Earth.

In 1986, scientists sailing in the Pacific Ocean made an astonishing discovery. In sediments collected from 850m below the seafloor, they identified that microbes were living and thriving in an environment not previously known to contain life. This discovery has spawned a new field of research on the "deep biosphere" with researchers exploring how life persists and evolves at hostile temperatures and pressures. With estimates that the sub-seafloor may contain as much as two-thirds of the Earth's microbial population, research today focuses on understanding the importance, or lack thereof, of this community to the Earth's systems. This presentation will focus on the current state of knowledge with respect to the deep biosphere and the major questions being addressed in this field, such as what are the nature and extent of life on Earth? What are the physico-chemical limits of life on Earth? How metabolically active is the deep biosphere, and what are the most important redox processes? What are the dispersal mechanisms for life in the deep biosphere? How does life evolve in deeply buried geological deposits that can occur more than a km beneath the ocean floor? What is the influence of the deep biosphere on global-scale biogeochemical processes?